A fuel cell is a power generation system for producing electrical energy through electrochemical redox reaction of oxygen and hydrogen in a hydrocarbon-based material such as methanol, ethanol, or natural gas.
A fuel cell can be classified into a phosphoric acid type, a molten carbonate type, a solid oxide type, a polymer electrolyte type, or an alkaline type depending upon the kind of electrolyte used. Although each of these different types of fuel cells operates in accordance with the same basic principles, they may differ from one another in the kind of fuel, the operating temperature, the catalyst, and the electrolyte used.
Recently, a polymer electrolyte membrane fuel cell (PEMFC) has been developed. The PEMFC has power characteristics that are superior to conventional fuel cells, as well as a lower operating temperature and faster start and response characteristics. Because of this, the PEMFC can be applied to a wide array of fields such as for transportable electrical sources for automobiles, distributed power sources such as for houses and public buildings, and small electrical sources for electronic devices.
A PEMFC is essentially composed of a stack, a reformer, a fuel tank, and a fuel pump. The stack forms a body of the PEMFC, and the fuel pump provides fuel stored in the fuel tank to the reformer. The reformer reforms the fuel to generate the hydrogen gas and supplies the hydrogen gas to the stack. Fuel stored in the fuel tank is pumped to the reformer using power which can be provided by the PEMFC. Then, the reformer reforms the fuel to generate the hydrogen gas, and the hydrogen gas is electrochemically reacted with oxygen in the stack to generate the electrical energy.
Alternatively, a fuel cell may include a direct methanol fuel cell (DMFC) in which a liquid methanol fuel is directly introduced to the stack. Unlike a PEMFC, a DMFC does not require a reformer.
In the above-mentioned fuel cell system, the stack for generating the electricity has a structure in which several unit cells, each having a membrane electrode assembly (MEA) and a separator (referred to also as “bipolar plate”), are stacked adjacent to one another. The MEA is composed of an anode (referred to also as “fuel electrode” or “oxidation electrode”) and a cathode (referred to also as “air electrode” or “reduction electrode”) that are separated by a polymer electrolyte membrane.
FIG. 1 is a schematic view showing an operating state of a fuel cell 1. The fuel cell 1 includes an anode 3, a cathode 5, and a polymer electrolyte membrane (or polymer membrane) 7. As shown in FIG. 1, when fuel such as hydrogen gas is supplied to the anode 3, an electrochemical oxidation reaction occurs to ionize and oxidize the fuel into protons (H+) and electrons (e−). The polymer electrolyte membrane 7 permeates the ionized protons to the cathode 5, but the polymer electrolyte membrane 7 does not permeate the electrons. The electrons are transmitted to the cathode 5 though an out-circuit (not shown). The transmitted (or permeated) protons (H+) on the cathode 5 are electrochemically reacted with oxygen contained in an oxidant on the cathode 5 to generate reaction heat and water. The electrical energy is generated by the transmittance of the electrons through the out-circuit. The chemical reactions in the fuel cell 1 may be illustrated by the following reactions:                Anode reaction: H2→2H++2e−        Cathode reaction: 2H++½ O2+2e−→H2O        
A polymer membrane-electrode assembly (MEA) is composed of a solid polymer electrolyte membrane (e.g., the membrane 7 of FIG. 1) and an electrode layer including catalysts supported on carbon. The polymer electrolyte membrane can be fabricated using a perfluorosulfonic acid ionomer membrane such as Nafion® (by DuPont), Flemion® (by Asahi Glass), Asiplex® (by Asahi Chemical), and Dow XUS® (by Dow Chemical). The electrode layer including the catalysts supported on the carbon can be fabricated by binding the electrode substrates such as a porous carbon paper or a carbon cloth with a carbon powder carrying pulverized catalyst particles such as platinum (Pt) or ruthenium (Ru) using a waterproof binder.
Conventional polymer membranes such as Nafion® have good proton conductivity (or proton permeability), good anti-corrosiveness, and good chemical resistance, but they are also high in cost and may allow methanol to crossover. Further, the conventional polymer membranes require a separate humidifier for humidifying the membranes because a supply of water is needed to permeate the protons (H+) through the membranes. Because of this, additional devices and spaces are needed. In addition, the conventional polymer membranes also have problems in that the needed water (moisture) is evaporated when the conventional polymer membranes are operated at a high temperature, thereby deteriorating their proton conductivity (or proton permeability).